Several types of instrumentation are presently available for elemental analyses. The four most predominant types are graphite furnace atomic absorbtion (GFAA), inductively coupled plasma optical emission spectrometry (ICP-OES), glow discharge mass spectrometry (GD-MS), and inductively coupled plasma mass spectrometry (ICP-MS). Each of these instruments with their associated techniques has its advantages and disadvantages in turn. There is no one form of instrumentation which is superior to the others in all respects.
GFAA is an optical technique. With it, an atom in its lowest energy state (the ground state) can absorb a specific wavelength of light which corresponds to the difference in the electronic energy levels of the excited and ground states of the atom. The more atoms of the particular analyte element which are present in a beam of this specific wavelength of light passing through the atoms, the greater will be the absorbance of that specific wavelength of light, and hence, the more absorbance will be measured. The amount of absorbance measured corresponds to the concentration of the analyte element in the sample. Principles of operation of atomic absorbtion have been described by J.W. Robinson in "Atomic Absorbtion Spectrometry," J.E. Cantle (Ed.), New York: Elsevier Scientific Publishing Co., pages 1-5 & 22-23, 1982. In GFAA, the sample is atomized in a resistively heated furnace. To absorb their characteristic wavelengths of light, analyte atoms must be free atoms, i.e., in the gas phase. See, M. Cope, G. Kirkbright & P. Burr, Analyst, 107, 611 (1982).
The main disadvantages of GFAA are its slow analysis rate, poor sensitivity with certain elements, and limited dynamic range. It is a slow technique because a different light source must be brought into use for every element to be analyzed. Its sensitivity varies according to how strongly a particular element absorbs light. For some elements, the absorbtion is very strong, but for others it is quite weak. For the weakly absorbing elements such as phosphorus and sulphur, the poor sensitivity results in very bad (high) detection limits. Its linear range of response is very dynamically limited. In general, its absorbance is directly proportional to analyte element concentration for solutions ranging from one part per billion (1 ppb) to 100 ppb. This represents two orders of magnitude. Above this range the absorbance begins to taper off and is no longer linearly related to concentration.
ICP-OES is also an optical technique. The ICP is a plasma which is maintained by the coupling of a radio frequency (RF) field with a stream of argon gas. It has been described by J. Olesik in Anal. Chem., 63(1), 12A (1991). Its temperature of 6000 degrees Celsius or higher is hot enough to atomize almost any substance. It is also hot enough to excite ground state atoms to a higher energy level, from which emission of characteristic wavelengths of light can occur as the excited atoms settle back toward less energetic levels, again, the least energetic of which is the ground state. The intensities of these emissions are related to the concentration of analyte atoms in the sample. In ICP-OES, emissions are resolved and quantified by a spectrometer to include a detector such as a photomultiplier. See e.g., D. Beauchemin, Spectroscopy, 7(7), 12 (Sep. 1992).
ICP-OES has two main disadvantages associated with it relative to these other known techniques. It is subject to interferences because of the large number of emission lines inherent to atoms, which result in congested spectra. Also, it is not as sensitive as most of the other techniques.
GD-MS operates by ionizing the surface of a solid sample and analyzing the ions by mass spectrometry. A high voltage charges the sample in a low pressure argon plasma in order to accelerate argon ions toward the sample surface. The argon ions impact with the surface of the sample, dislodging free atoms, which become ionized as a result of further collisions in the plasma. In GD-MS, the plasma interfaces directly with the vacuum system that contains a quadrupole mass spectrometer. See e.g., R.K. Marcus, Spectroscopy. 7(5), 12 (Jun. 1992).
Disadvantages of GD-MS include that its sample must be electrically conductive, or made to be so, for it to be atomized. This is not always practical or even possible. At the very least, nonconductive samples such as biological samples and so forth must be mixed with a metal agent to make them conductive. This agent can be a source of contamination of the sample.
ICP-MS also generates ions from the sample and analyzes them by mass spectrometry. The atomization and ionization of the sample occurs in an inductively coupled plasma which is essentially the same as that in ICP-OES. The plasma is hot enough not only to atomize the sample but also to ionize most of the analyte atoms. In ICP-MS, however, the plasma generator is mounted so as to orient the plasma horizontally, and the plasma is directly interfaced to the inlet of a vacuum chamber for ion analysis. See, R. Houk, Anal. Chem., 58, 97A (1986).
Disadvantages of ICP-MS include that interferences and a form of ion suppression are often unavoidable. This is caused by the atomization and ionization steps which occur in the same region, a wet argon plasma. The interferences result in relatively poor detection limits for important atoms such as Si, S, P, Cl, K, Ca, Sc, V, Mn, Cr, Fe and Se.
Another known mass spectrometric system employs a Knudson cell as its sample source. The cell is positioned internally, and is heated slowly under the vacuum of the instrument until its solid phase sample contents come into thermal equilibrium with its vapor phase and the cell. The vapors effuse through a small opening in the cell and travel to the ionization region as a molecular beam. The mass spectrometer measures the ion current resulting from the sample. The purpose of this instrument is to establish the vapor pressure of the atomic or molecular species inside the cell rather than to determine elemental analysis of a sample. See, J.R. Majer, "The Mass Spectrometer," London: Wykeham Publications, Ltd., Crane, Russak & Co., Inc., page 71, 1977.
Further art concerning such instruments, parts thereof, and so forth, and improvements in them is known. See e.g., Kraus et al., U.S. Pat. No. 3,619,839 (Nov. 16, 1971), for an electrically heatable cylindrical sample container; Calcote et al., U.S. Pat. No. 4,278,441 (Jul. 14, 1981), for a flame sampling apparatus and method; Pink, U.S. Pat. No. 4,673,656 (Jun. 16, 1987), for aerosol production in inductively coupled plasma emission spectroscopy; Castleman, U.S. Pat. No. 4,828,800 (May 9, 1989), for a system for trace gas detection; Forster et al., U.S. Pat. No. 4,916,077 (Apr. 10, 1990), for a method and apparatus for oxidative decomposition and analysis of a sample; Williams et al., U.S. Pat. No. 5,135,870 (Aug. 4, 1992), for laser ablation/ionization and mass spectrometric analysis of massive polymers. This is not a technique for elemental analysis. See also trade literature for the following instruments: Model 5100 PC and Seeman/5100 PC atomic absorbtion spectrometers (Perkin Elmer, 1987); SpectrAA-300/400 atomic absorbtion spectrometers (Varian, 1987); ICAP 61E plasma spectrometer (Thermo Jarrel Ash, 1989); TS SOLA modular mass spectrometer system for multi-element analysis of liquids and solids, ICP-MS & GD-MS in one instrument (Turner Scientific).